Stone fruit growers advised to watch for plum pox virus

Plum pox virus is an exotic, invasive pathogen spread by aphids. It can cause premature fruit drop, impact fruit quality and damage the tree.

PPV was first identified in Europe in the early 1900s, and it is now present in all major fruit growing countries except for the U.S., Australia, New Zealand and South Africa.

In 1999, PPV was detected in Pennsylvania, and it took 10 years and nearly the complete destruction of the state's stone fruit sector to eradicate the disease. There have also been outbreaks in New York and Michigan, but they also have been eradicated.

A year after it was found in Pennsylvania, PPV was detected in Ontario, Canada. In 2012, after a 10-year eradication effort and the removal of almost 275,000 stone fruit trees, Canada replaced its eradication effort with a PPV monitoring and management program.

While California has yet to find the disease, the University of California, Davis, recently held an international conference on PPV. Gary Obenauf, director of research for the California Dried Plum Board, was a speaker at the conference.

According to Obenauf, California has approximately 973,800 acres of stone fruit that would be susceptible to the virus. Further, he continued, "Unlike Pennsylvania, the California stone fruit industry is characterized by very large commercial orchards grown in close proximity under ideal environmental conditions for aphid and virus spread."

PPV will impact all stone fruits, including peaches, plums, nectarines, prunes, cherries, and apricots as well as almonds.

The PPV virus has the potential to cause anywhere from 10 percent to 100 percent damage, depending on cultivar susceptibility, Obenauf said.

"It's felt that PPV will eventually get here. Of course, we are hopeful that it doesn't," Obenauf said. "Part of the problem we have right now is that the Canadians have essentially given up on trying to eradicate it. So that means we're going to continue having the inoculum in close proximity to us, and as long as it's there, it's always a possibility that it's going to move out this direction."

If PPV came to California, it would mean a lot of dollars, a lot of acreage and a lot of growers going out of business, Obenauf said.

"Right now, our major efforts are trying to do early detection of the virus. If it does come in, we need to find it quick, and quarantine it, and keep it from spreading," Obenauf said.

PPV is primarily spread by aphids feeding from infected trees, then moving from tree to tree transferring the disease, Obenauf said.

Aphid control will help reduce the spread of the disease, Obenauf said.

"We have several materials that give very good control right now," he said.

There are also some predators for aphids, Obenauf said.

"What we have is not that effective. We've done a lot of research on trying to find predators where the aphids were native in Europe and Asia, and we brought some of those in, but they haven't got established to the point where they're effective by themselves," he said.

Certainly, orchards are a concern for PPV, but nurseries are even more of a concern, Obenauf said.

"If you get the virus in a nursery, obviously you could spread it around a lot quicker that way," he said.

Ralph Scorza, a research horticulturist with the U.S. Department of Agriculture Agricultural Research Service, also spoke at the conference.

Scorza said what can help growers is publicity, such as knowing what the symptoms are and what to look for. Growers should always question the source of budwood, he said.

"Those kinds of things, I think, are going to be important for growers to be aware, keep up with what might be happening in terms of plum pox virus," Scorza said, adding that if PPV ever comes in, a grower is probably going to be the person who identifies it.

The quicker PPV is identified, the quicker it can be eliminated, he said.

Research is being done for resistant varieties, Scorza said.

"Knowledge is power,"he stressed. "There are very few varieties that are really resistant to plum pox virus. There's some resistance in a couple of apricot varieties. It's a high level resistance.

"There are some varieties that are tolerant. That means they'll get the virus and they won't show many symptoms. But those are the kinds of trees that are the scariest because you don't know they have the virus in them, and meanwhile the virus is spreading throughout your orchard," Scorza said.

"We've been working on genetically engineered resistance, where we actually use a little piece of the virus that's not infectious and we insert that into a plant, and that basically stimulates the plant's defense mechanisms against plum pox virus," Scorza said, adding that this method has been tested in Europe since 1996, and those trees have never been infected with the virus.

"There are some almonds that appear to be resistant, and there are some almonds that appear to be susceptible," Scorza said. "My guess is that researchers will find out that there are some almonds that are resistant. Are those the varieties that the growers want to grow now? That would be a question."

Scorza said a lot of research needs to be done, specifically on the almond, adding there are reports from Europe that basically say almonds are susceptible, but there are also reports where almonds have the virus and don't show symptoms.

That's a very difficult situation because if the virus gets into almonds, good symptoms won't be seen, and the aphids could pick up the virus from almonds and carry it to stone fruits, Scorza said.

Deborah Golino, director of Foundation Plant Services at UC Davis, helped put on the conference.

"The research is clouded about whether or not almond can be a host for this virus," Golino said.

"We feel it's very urgent to get some clear research on almond, and in particular the cultivars of almond that we grow in California, because if this virus came in and almond proved to be a host, because of the huge acreage of almonds it would be devastating for other stone fruit people," Golino said. She said Tom Gradziel, an almond breeder at Davis, will be working with a group in Spain to research this.

"The virus would be extremely difficult to control if it gets to California, so we need to keep people alert to the dangers of bringing unregulated plant material, and we need to keep people alert in terms of reporting symptoms if they see anything and contact the county agricultural commissioner," Golino said.

Education on PPV, especially to growers, is important, but everyone should know the signs and symptoms, Scorza said.

"I think money would be well spent on looking at ways to not have PPV come in, and if it comes in to be immediately recognized and spotted, so the trees can be removed, and we don't have big losses," Scorza said.

(Kathy Coatney is a reporter in Corning. She may be contacted at kacoatney@gmail.com.)